This invention relates generally to an automatic coordinate determining device, and more particularly to an automatic coordinate determining device whose upper surface of a coordinate tablet is electrostatically shielded. A conventional automatic coordinate determining device is constructed as shown in FIG. 1. Numeral 1 is a coordinate tablet formed with an insulating plate 2 having conductive lines Xn, Ym. Numerals 3X, 3Y represent scanning means for scanning respective conductive lines Xn, Ym. Numeral 4 is a pencil shaped probe having an excitation winding 5 for generating the inductive signal against the conductive lines Xn, Ym. Numeral 6 is a crystal oscillator which coacts with an amplifier 7 to form an oscillatory signal generator to magnetize the winding 5, and the alternating inductive frequency of the oscillatory signal generator is about 300 KHz. Numeral 8 is a pen switch for supplying a signal to an operation control means 11 when operates at more than a predetermined pressure. Numeral 9 is an amplifying filter means for processing the wave form of the induced signal 1 from the conductive lines Xn, Ym. Numeral 10 is an AD converter for converting the output of the amplifying filter means 9. A scanning detecting means is made up of conductive lines 3X, 3Y, the amplifying filter means 9, and the AD converter 10. The operation control means 11 includes a data memory and CPU (not shown) for operating the coordinate of the probe 4 with the output of the AD converter 10, and controls all the external input and output signals. Numeral 12 is an external device which has output and input terminals connected to the operation control means 11. Numeral 13 is a scanning address register which is controlled by the operation control means 11.
In the case of actually using the coordinate determining device as shown in FIG. 1, a hand H which holds the probe 4 must be set above the plate 2 having the conductive lines Xn (Ym), i.e. on the cover member 2a (made of an insulating material) as shown in FIG. 2. In this case, floating capacitances Cs1 and Cs2 are formed respectively between the winding 5 and the hand H, and between the hand H and the conductive lines Xn (Ym). Therefore the electric potential of the capacitance Cs1 is vibrated by the electromagnetic wave generated by the alternating magnetic field from the winding 5. This electrical vibration is coupled with the conductive lines Xn (Ym) electrostatically via the hand H and the capacitance Cs2. Accordingly, the conductive lines Xn (Ym) generate both the electromagnetic induced signal by the winding 5 and the electrostatic induced signal by the hand H.
The scanning signal wave form in the above case is shown in FIG. 3, and the original scanning signal waveform by the electromagnetic induction is levelled up in proportion to the electrostatic induced voltage Vo. Generally, the maximum signal voltage Vp is about 6 V, while the electrostatic induced voltage Vo can reach to the extent of 2 V. Therefore, the voltage of the secondary peak P1 or P2 which is not detected in the normal state can exceed the threshold voltage Vth. Then the operation control means 11 may misjudge the secondary peak P1 as the maximum signal when the scanning is excuted from the left side in FIG. 3, and operates a mistaken coordinate of the probe 4.